Since ものなら　means something that you think isn't likely/possible to happen, I believe the first part of the sentence does not imply the listener is the subject, but the 'that person' in the next part of the sentence instead.

So basically it is saying: "You should be careful because it's impossible to get that guy to help with (my) work, but he uses it* very often as an opportunity to ask for favors."

*it refers probably to asking that person for help, or possibly something else given the context.

From JGram: ～う／ようものなら：　もし～ようなことをしたら．．．（大変なことになる） Once if (one) happen to ... , then (bad consequence)."

So the first part of the sentence does not imply the listener is the subject, but the 'that person' in the next part of the sentence instead.

So basically it is saying: "You should be careful because if you help that guy out once, he will take advantage of that (act) and ask for favors very often."