Villain is 25/14 after 300 hands. Playing around 38% of his hands from the BB and cbetting wide.

Stacks:
UTG - Hero ($5.85)
BTN - BTN ($5.00)
SB - SB ($7.52)
BB - BB ($6.01)

Preflop: ($0.07, 4 players) Hero is UTG with A 8
Hero raises to $0.14, 2 folds, BB calls $0.09

Flop: 5 9 9 ($0.30, 2 players)
BB bets $0.15 Hero?

Call flop and re-evaluate turn. Can obviously continue on turns with any diamond/6/7/8/A and on blank turns, could potentially raise if villain donks again and give up if he calls.

Yep, call and let's see the turn.

I'm going to copy-paste a good reddit post I liked and bookmarked. These are not my words but I agree with what the guy is saying and in my experience, it's close to what you see people donk flops. I think you'l find it a good read.

"Wide linear range: Typically high donk stat (especially 40%+), low check/raise, high check/fold to cbet.
Answer this player by calling most of your middling hands, raising your best hands.
You rarely want to raise as a bluff as they already have told you they have something (and these players are generally very sticky).
But most of your money will be made by attacking this players very weak checking range. He is capped, so feel free to bluff and value bet very thinly.


Middling mediocre hands and draws: In my experience, this is the most common type of range at micros.
Probably has a high donk stat, combined with higher check/raise and low check/call.
This player will check/raise their strong hands and donk their middling hands to "find out where they are at".
In this case, their donk range is capped, so a good response is raising frequently.
Against many players, this also makes their check/call flop range very weak, so you should consider barreling turn frequently.


Weakest part of range: You see this most often with players that are basically never folding flop.
If you see a guy that donks 30% check/raises 20%, check/calls 45%, they will frequently be donking their very worst hands only.
They don't want to call a bet with such a hand, but folding is out of the question to them of course, so they bet hoping to win the pot immediately.
You should respond to this by very rarely folding. You can raise wide for value and as a bluff and call some quite weak holdings (A/K high should call)


Tight linear range: You see this most often with loose passive players (for example like 30/5 preflop) that have a relatively low donk stat like 10-15%
and very low c/r stat. Normally a donk from this kind of player means "I have a strongish made hand and I don't want to give a free card, so I bet".
I'm playing much tighter against this strategy than the others.
It's worth noting that you can still probably raise TPTK+ for value because a lot of their range will be TP with decent/mediocre kicker.
Again, this player caps his range when he checks, so you should feel free to bet frequently for thin value or as a bluff.


Polarized range: I rarely see this, as the recreational players that like to donk a lot tend to prefer a more linear strategy.
One common situation where you might see this is when a reg donks into you a multiway pot with a fish.
Regs are incentivised to bet their strong hands to get value from the fish, knowing that you will not be frequently cbetting as a bluff.
And if they are good, they probably won't want to have a range of only strong made hands,
so they will include a few strong draws that don't mind bloating the pot as well."