This sentence comes from a question in my JLPT practise book that is part of an explanation of the use of ものなら
:
一度{いちど}仕事{しごと}を手伝{てつだ}おうものなら、それをいいことにあの人{ひと}は何度{なんど}でも頼{たの}んでくるから、気{き}をつけたほうがいい。
My fractured translation is, "If helping out a little is something you intend / that's something good / asking that person's help / you should take care."
As you can see, though, I can't make the connection between asking for help and why one should take care.
What exactly is this sentence saying, and how does it relate to the use of ものなら
?