IF it is EVER appropriate to approach someone with something like that a simple, "You know I am always here and, if adoption is something you would consider I would gladly raise the baby." Nothing more than that.
There's no evidence that the OP DID say anything more than that. As per post 6 of this thread, she was very restrained about her feelings and beliefs out of consideration for the mother's feelings. She made a deliberate effort not to "shame" her, as you put it.
Honestly, I don't get the "under no circumstances must you hint that you think abortion is a bad thing" mentality in this thread. If the mother is so mentally fragile that she'll have some kind of meltdown at the realisation her boyfriend's mother is pro-life, she shouldn't be making a decision as momentous as whether or not to terminate. The OP obviously recognises that the mother has a legal right to terminate, but that does not mean she (the OP) is somehow obliged to believe that it is a moral choice, or to pretend that she does.
Obviously being unkind to the mother wouldn't be helpful to anyone, but why should she have to "support their decision"? What does that even mean? If it means "refrain from locking the mother in a room for nine months", well, yeah, duh. But if it means "pretending that she feels either choice is equally morally valid" when she clearly doesn't, well, no. She's allowed to be true to her beliefs. I've come across a kind of "It's OK to be pro-life deep down inside as long as you still drive your friends to the abortion clinic" mentality before, and I think I see it on this thread; and I don't get it. It's like saying "It's OK to be opposed to adultery as long as you help your friend set up assignations and don't breathe a word that you think her cheating is wrong; otherwise you're interfering with her freedom of choice" - that being, apparently, the cardinal sin these days. (Yet, here on MDC, people are happy to use emotional arguments and persuasive techniques to persuade people not to do things within their legal rights, such as circumcising their babies, formula-feeding, whatever; even though these things, too, impact on a woman's freedom of choice and how she uses her body!)
Not to mention that legal "freedom of choice" does not mean "a choice made without considering all aspects of the question". Heck, at 18 the mother's decision-making faculties aren't fully developed. As parents (or rather, relevant adults) we recognise that in most scenarios, giving our opinion - generally without coercion, one hopes - on questions ranging from "should I quit Uni and become a rock star?" to "my boyfriend left me, should I torch his car?" to "is marijuana OK?" Why is it OK for a person to express opinions (even strong, even morality-based) opinions on subjects like that, but suddenly taboo when it comes to something like abortion (which a huge percentage of women regret, incidentally)? Why is "Marijuana can damage a growing brain, is against the law, and I would be disappointed if you did it" OK to say, but "There is a living baby in you which I believe has a soul, and I would be very sad if he didn't get to live" so sinister?
OP, thinking about your offer to adopt, I can see why the mother would be reluctant. No offense, but if I were a pregnant Wiccan, I probably wouldn't want my child brought up by a conservative Christian either, you know? What she might (possibly) prefer would be an offer simply to help in any way you can. She might want to bring up the baby herself, but be worried about going to Uni with a baby - you could offer to watch the baby during the daytime. She might be worried about money - you could offer to help get her set up in a house, or whatever. If you have a good relationship, maybe you could find out specifically why she doesn't want to have the baby - and if it's anything you can help with (assuming you're willing to take it on), you could (gently, tactfully) offer to help. People don't abort for no reason, and lack of support (financial, child-care, emotional, etc) is often a big factor; so offering to help, but not in a way that would take ownership of her child or control over her/his bringing-up, might be something she'd appreciate.
What does your son feel about all this? Would he step up to his responsibilities as a dad if she continued the pregnancy?