What would you call a person who helps you only to later boast about it or repeatedly, emphatically mention it during a fight? [duplicate]

I doubt there's a term for exactly OP's context, but to me it's an example of the (primarily AmE, possibly non-PC) idiomatic usage. From Wikipedia...

Indian giver
a person who gives a gift and later wants it back, or something equivalent in return

The original assistance wasn't in fact freely given, with no strings attached, even though it might have seemed so at the time. The "not-so-good Samaritan" seeks to be repeatedly "repaid" by aggrandizing themselves at your expense for the indefinite future.

I would describe such a person as "petty":

mean or ungenerous in small or trifling things: a petty person

showing or caused by meanness of spirit: a petty revenge

(http://dictionary.reference.com/browse/petty)

First they give you something (help or assistance), then they essentially cancel it out by being petty about it.