A common response whenever these threads come up is that age gap relationships must be strange because either you are from a different generation and therefore can't have anything in common or there is an imbalance in power.
The thing is I know lots of same age relationships where they have nothing in common. And surely if you were having a relationship with someone 'having things in common and enjoying the same stuff' would be a basic first step. You wouldn't have a relationship with someone you didn't have stuff you could share with would you.
As for imbalances of power - you only have to read this board to see that there are plenty of imbalances going on. I was married to a man I had little in common with - and in fact many opposing views. We did nothing together. He showed me little love, affection or interaction.
Now I have a large age gap. We are perfect for each other. It's lasted four year so far, he's taken on my children (and more energetically and patiently than me most days) and we are perfectly suited.
No my parents don't mind. They see a man who treats me well. No my friends don't laugh. They see a man they also have things in common with. Others actively encouraged us to get together.
Yes he could die. However I know far too many friends with a same age partner who died young. And I'd rather have a few years of a beautiful relationship that the 60 odd years I might have had of misery with my ex husband.
Yes he will get old (if he doesn't die first). Yes I will care for him. That's what you do with people you love. It's not about 'benefits'. If I was the same age as him likely I would still care for him. Just I'd be old and needing care too.
Would I have a relationship with any man his age? No. Would I have things in common with most men his age? No. But him? Yes.
I might be young (37) to his older (66) but in terms of the things that matter to me - views on life, politics, how you treat other people - we are the same. We share all the same hobbies. He is active and kind and supportive. Yes he will age and slow down but frankly so will I. He has put up with me being exhausted from the children, ill for a long period of time and stressed from work. In fact he did more than put up with me - he cared for me during this time. I will do the same for him.
I am succesful in my career and earn the same money as him. I am not with him for the money. Or the power. There is no power imbalance.
We pay for things equally. We do equal housework. He has taken responsibilities for the children and spends time helping, playing, loving and teaching them. He cooks for us all. Is thoughtful. How lucky am I to have that? (especially when you read this board).
And as for the question I know nosey people want to know. He's fantastic in bed. Most attentive and skilled man I've ever known. Certainly no complaints there. It's more likely me who is too tired! 
As for 'clearly an issue if you can't get someone your own age' ... I have plenty of men my own age who showed interest when I was divorced. Many offers of dates. But why would I want someone else purely due to age when I literally can't fault my relationship (ok he sneezes way too loudly). Likewise - he has plenty of admirers and could easily find a woman his own age. But he wants me. Again - why would he over rule our relationship for age?
I just don't understand the assumptions. Are they based on personal experience? Or stereotypes? Yes there are the stereotypes out there - but there are across age groups!
It's about the individual surely. If she is having doubts about him, then support her. But give her advice based on him not his age, as that is irrelevant.