I don't understand how he is doing this. I must be missing something simple here.

Aren't I supposed to be able to know his cards? heh

Here are 2 raw HH's to show what I mean.
***** Hand History for Game 6650477289 *****
$100 USD NL Texas Hold'em - Wednesday, January 02, 17:56:10 ET 2008
Table Table 128369 (Real Money)
Seat 6 is the button
Total number of players : 6
Seat 1: andylauser ( $100 USD )
Seat 2: Timor83 ( $95.50 USD )
Seat 3: mistereddy ( $175.18 USD )
Seat 6: Hero ( $301.83 USD )
Seat 4: lucky_german_girl ( $283.14 USD )
Seat 5: captntappa ( $96 USD )
Timor83 posts small blind [$0.50 USD].
mistereddy posts big blind [$1 USD].
** Dealing down cards **
Dealt to Hero [ Qc Jh ]
>You have options at Table 128355 Table!.
>You have options at Table 126536 Table!.
lucky_german_girl folds
>You have options at Table 128355 Table!.
captntappa folds
>You have options at Table 130542 Table!.
Hero raises [$3.50 USD]
>You have options at Table 128355 Table!.
>You have options at Table 126536 Table!.
Timor83 folds
mistereddy calls [$2.50 USD]
** Dealing Flop ** [ 9s, 3d, Jc ]
mistereddy checks
Hero bets [$6 USD]
mistereddy calls [$6 USD]
** Dealing Turn ** [ Jd ]
mistereddy checks
Hero bets [$14.50 USD]
mistereddy calls [$14.50 USD]
** Dealing River ** [ 2d ]
mistereddy checks
>You have options at Table 125957 (No DP) Table!.
Hero bets [$21.50 USD]
mistereddy calls [$21.50 USD]
Hero shows [ Qc, Jh ]three of a kind, Jacks.
mistereddy does not show cards.
Hero wins $89.50 USD from the main pot with three of a kind, Jacks.
>You have options at Table 130542 Table!.

***** Hand History for Game 6650404870 *****
$100 USD NL Texas Hold'em - Wednesday, January 02, 17:37:54 ET 2008
Table Table 128369 (Real Money)
Seat 3 is the button
Total number of players : 6
Seat 1: andylauser ( $86.93 USD )
Seat 2: Timor83 ( $100 USD )
Seat 3: mistereddy ( $97.09 USD )
Seat 5: bethbentom ( $0 USD )
Seat 6: Hero ( $128.16 USD )
Seat 4: lucky_german_girl ( $314.74 USD )
lucky_german_girl posts small blind [$0.50 USD].
Hero posts big blind [$1 USD].
** Dealing down cards **
Dealt to Hero [ Ad As ]
bethbentom has left the table.
andylauser calls [$1 USD]
Timor83 folds.
mistereddy raises [$5 USD]
lucky_german_girl folds.
Hero raises [$15 USD]
captntappa has joined the table.
andylauser folds.
mistereddy calls [$11 USD]
** Dealing Flop ** [ 7d, 8c, 9h ]
>You have options at Table 130542 Table!.
Hero bets [$27 USD]
mistereddy calls [$27 USD]
** Dealing Turn ** [ 2h ]
Hero bets [$63 USD]
mistereddy is all-In.
** Dealing River ** [ Ah ]
Hero shows [ Ad, As ]three of a kind, Aces.
mistereddy does not show cards.
Hero wins $8.91 USD from side pot #1 with three of a kind, Aces.
Hero wins $193.68 USD from the main pot with three of a kind, Aces.

Hero is calling your bets (i.e. paying for the right to see your cards). If he's lost the hand, there is no obligation to show his cards.

This is not right. You are entitled to see at showdown. Havn't played Party for years so not sure how you find out. Is there a last hand button? email Party and ask.

Yeah I noticed this as well, they are always showed in last hand window, but not always in the HH file. Not sure why exactly, could be a bug or something on their part.

You can try to request specific hand history by email and see if it shows his hand there. (Party Main window>Game Rules&Options>Instant Hand history>write in game # and press send email)

From "Roberts Rules of Poker"...

Any player who has been dealt in may request to see any hand that is eligible to participate in the showdown, even if the hand has been mucked. This option is generally only used when a player suspects collusion or some other sort of cheating by other players. When the privilege is abused by a player (i.e. the player does not suspect cheating, but asks to see the cards just to get insight on another player's style or betting patterns), he may be warned by the dealer, or even removed from the table.

Note the word privilege - I.e. There is no obligation to show a losing hand. If you play in any live environment, you see this all the time. Why should online be any different?

Hi funny, are you proving my point or yours? This says you are entitled and all poker sites (that I know of) have an option to see it.

Anyways, the rule you state is a little ambiguous and I think it refers to the eligabilty in the following circumstance: 2 or more players remain after the river is dealt and there are no bets - 1st to act mucks before any other players cards are shown. I may be wrong but that is my understanding.

In the scenario of the OP - In a live game it is considered poor form to insist on seeing a mucked hand but you are entitled to do so and that is why all sites have the option.

You can just click at the top right corner and request the hand history where you'll be able to see his cards. Also, if you use PokerAce Hud, that'll show it too. Btw, I noticed a very good player on your table

I think, that party just stopped showing mucked hands recently, because I cant find them even in the HH.

Not true, just checked to confirm. Also why are some posters posting rules of showdown etc.? Not all sites do it the same way. If you don't know how party does it, don't comment.

At party if the hand goes to showdown you can see mucked hands 99.99% of the time. Sometimes, as in OP's examples, you can't see the hand for some reason. It happens so rarely that I have yet to find any pattern.

If you can't see the hand in the instant hand history window then pahud won't be able to show the hand and it also won't show if you request the hand again by email.