Please accept my help! What do I do?

I haven't checked things out here for a while, but this is always a good place to come for some advice on stuff like this. Please help if you can! I'm sorry it's so long....it's a long story.

I have a friend that had a stroke several years ago and has been bedridden ever since. Once/month he needs to have a blood test. This was a huge ordeal and went on for a while before I found out about it. The poor guy's wife had to arrange for a special lift to be brought to the house and then have a few people over to help get him into a special chair and then into a special van just to go to the hospital for a simple blood test...then back again. It would be a 3+ hour ordeal for them. Well, I work in a hospital lab and as soon as I found out they were doing this I offered to just come to their house and draw his blood test. It was truly my pleasure to do it for them. I consider them friends and I hated to think of all the trouble they were going through for such a simple thing. I knew they were grateful and that's always been enough. I don't know why but for some reason lately the wife is always wanting to give me money. If I don't accept it at the time I'm there then she will send it later in a card. These are older people on a fixed income and have NO money. I've told her it's my pleasure to help them and nothing like that is necessary, but she keeps doing it. I feel terrible accepting it because I know how much they need it and I can never bring myself to spend it. I just wish that she would accept my help without feeling the need to pay me. They just can't afford it and I'm so happy to just help them. Does anyone have any suggestions? Home health care is not an option for them for some reason I don't understand and I've kindly explained to her not to give me anything several times. I'm out of ideas. I can't stand to accept their money for this. Please help....