Should you counsel a soldier for being a simp?

You could have a verbal counseling with the Soldier on his/her choice in meeting people and worn against sending money to strangers. Typically doesn't change their mind. You might have a better chance influencing a change if someone else had been through it could talk to this Soldier.
As to military counseling, you could counsel on financial obligations/debts (AR 600-15 and AR 608-99) and the consequences associated with debts (UCMJ, loss of security clearance).

If what he is doing is affecting good order and discipline, then counsel him. If he is able to meet financial obligations and isn't affecting the work place, then leave him be. You cant tell him how he spends his money or who he talks to. You have to tread lightly and avoid the perception of targeting. As for what MAJ Garbarini stated, 600-15 and 608-99 can only be mentioned if the soldier has shown financial irresponsibility or with the notification of indebtedness through the S2 or company.

The minute their performance or finances decrease, it becomes your problem period. It a bitch, but you may have to wake him up, because this can effect his unit as well as himself. That's you job, train, mentor, listen, advice, counsel!