If I understand correctly, you are assuming that this player is a reg because you saw him at another table?  I see you don’t use a hud, but since you have HM2 can you see your hand history and see how many hands you have played against this opponent?(I use PT4 and know nothing about HM2) That would help you get a better idea if they are a reg or not.  Also, when you are done your session you can use a tracking website (sharkscope, poker pro labs, official poker rankings) to see this opponents results to help get a clearer picture of whether they are a reg or a rec player.

How did this player accumulate such a large stack?  Have you been playing with this opponent for a while or were you just seated with them recently?  Seeing how the opponent accumulated their stack can be beneficial information.

I don’t think many regs donk bet the flop.  If a reg has something strong on the flop, I think they would elect to check raise.  So I am starting to think that this player might not be a reg.  When people donk bet into me, I like to reraise them especially when I have something even if it’s middle pair here to keep up the preflop aggression because if they are semi-bluffing a draw they will probably fold.  Since the 6h completes a straight draw I might be more inclined to try and just check down to SD.  I would definitely fold here to the 5d on the river.