A9s flops flush draw - how should villain have played?

[morberg]

Party Poker 0.5/1 Hold'em (8 handed) converter

Preflop: Hero is UTG with 9[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/heart.gif[/img].
Hero calls, UTG+1 calls, MP1 calls, <font color="#666666">4 folds</font>, BB checks.

Flop: (4.50 SB) 4[img]/images/graemlins/heart.gif[/img], 2[img]/images/graemlins/heart.gif[/img], K[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(4 players)</font>
BB checks, <font color="#CC3333">Hero bets</font>, UTG+1 calls, MP1 calls, BB folds.

Turn: (3.75 BB) Q[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">UTG+1 raises</font>, <font color="#CC3333">MP1 3-bets</font>, <font color="#CC3333">Hero caps</font>, UTG+1 calls, MP1 calls.

River: (15.75 BB) 3[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">UTG+1 raises</font>, MP1 calls, <font color="#CC3333">Hero 3-bets</font>, <font color="#CC3333">UTG+1 caps</font>, MP1 calls, Hero calls.

Final Pot: 27.75 BB

Results in white below: <font color="#FFFFFF">
Hero has 9h Ah (flush, ace high).
UTG+1 has 7h 6h (flush, queen high).
MP1 has Kh 8h (flush, king high).
Outcome: Hero wins 27.75 BB. </font>

I'm pretty satisfied with my own play; it has to be -EV to slow down with an A high flush although a straight flush is possible on the river, right?

What I was thinking about is the play from MP1, can he fold anywhere in this hand and keep his sanity? UTG+1 just seems nuts in my book, but I can see myself in a position similar to MP1's.