What is the most shameful thing you have ever done to have or while having sex? For example, I screwed my best friend's wife the night we had a memorial service for him. It was a month after he died in an automobile accident. She came on to me as she had while he was living and I had not yielded to her. Unfortunately, I yielded this time. There were extenuating circumstances, but it was still shameful on my part.
I don't want to sound insensitive here but is a widow supposed to live a life of celibacy after her husbands death? Is she off limits from then on out? Everyone's grieving process is different. It's good that you two didn't do anything when he was around, thats as it should be. But I don't see anything you should be ashamed about.
Don't believe you did a thing wrong - she came onto you and I hope you both enjoyed the moment - if so go back for more