What a poor choice of wording in a time of need.
Honestly, I used to say it myself, but I myself have been in a situation where people have said "let know if I can do anything for you." Now being on the flip side, I find that it's quite annoying and I honestly am appalled that I would not even know the affects one sentence could have.
If you're like me, you're not one to ask for help and as a woman you're more than likely not either. Instead of asking for someone to "let you know", why don't you just do like Nike, JUST DO IT! DO what you THINK this person would like. IT means more than just saying "let me know if I can do anything for you."