Anonymous wrote:I think it’s likely to ruin the relationship no matter how you say it unfortunately.
I strongly disagree with this. If she's a reasonable person, I don't think it will. I have been asked to be a guardian, and I've asked, and while both were yesses, I think a no would have gone over fine. Now, obviously, it's awkward, and I do think your approach matters. "Nah" or "Your kid's too hard" will not go over well.
I would go with something like this (ideally in person/over the phone):
Hey - I am so, so honored that you would ask me to be a guardian for your son. I've put a lot of thought into this, but I don't think I could do a good enough job. Should the worst happen, you need someone who can be a rock for your son, and be there for whatever he needs in such an awful time. But I have very little/no experience with teenagers, and I generally don't feel like I relate to them well, plus I'm honestly overwhelmed with just the children I have (NOTE: don't say this if you're thinking of having more kids!) Your son deserves someone who can truly rise to this occasion, and I don't think that's me. I know you want what's best for him, so I think you should ask someone else. Of course, if anything happened, I would pitch in however I can, and would be happy to help out, but I don't think I have it in my to be a stellar primary caregiver for him.