However immediately a thought occurred to me that maybe I had done something really bad by rejecting her offering, and I felt really bad for not taking it. In theory, although I am not any kind of great practitioner, lama or Guru, I aspire to be a better practitioner and have taken Bodhisattva vows and so forth. On some level I wondered if rejecting her offering had eliminated a potential cause for her to accumulate good karma and merit since she was making an offering to a Buddhist (even if she didn't know it), and my main concern is that I might have done just this.
So my question is this: Should we always accept offerings from people even if we know they need it more than us simply to allow them to generate karma and merit? Since she does not know I am a Buddhist and her intention was simply to return kindness, was any harm actually done by my rejecting it? I know if I could go back in time I would accept it, simply based on my immediate reaction and awareness of how I might negatively affect someones ability to generate sources of merit and good karma. At the same time her intention was the same either way, it was a completely selfless gesture made by someone who had nothing in order to return kindness to a complete stranger who obviously didn't need it and on that level I can see how she still generated good karma and merit.
I once went to a fish release. However when we arrived at the docks there were no fish to buy. The Lama with us (Thubten Nima) said we had generated the same amount of beneficial karma regardless, as we had had the intention to purchase and free fish.
Thus, since your friend had the intention to give the gift to you and carried out the intention to the best of her ability, she generated the same amount as if you had accepted.