Winston Churchill summed this situation up perfectly. A man had been talking to him for some time then, when it was time to leave, commented that he had not mentioned his grandchildren. Winston Churchill's response: "and I am so glad you haven't."
I don't think any of us begrudge some proud grandparent talk and the odd photo here and there, but one's interest in other people's grandchildren is usually fairly limited and they don't feel that they are the best, most talented, most beautiful children in the world which, of course, you naturally do. The problem with some grandparents is that their grandchildren are the only topic of conversation they have and l wonder if you are in this category. It is boring at best and, in the case of someone who yearned to be in your position but it was not to be, is thoughtless and hurtful. If you truly value this friendship, l would limit the grandchildren talk to a few short anecdotes if you are asked. Otherwise, I would talk about other topics, especially those of particular interest to your friend. Keep up with current affairs, too. You may well find yourself feeling refreshed for the opportunity to think and talk about other things, too.