It is not something I would presume to do for someone else... but since you asked, here are some points that I would think are worthy of consideration:
You said maternal grandmother will not be available for childcare help. What about paternal grandmother? Is that an option?
They have been in a 3 yr relationship, is that right? Prior to the pregnancy, did they have plans for the future together, or just their own plans and nothing joint? Were they already drifting apart due to the difference in their ambition?
She is ambitious and he is not. What does that mean exactly in terms of support for the child? CAN he pay support in any meaningful amount? Daycare? Medical?
In almost all cases of voluntary adoption I think the most important thing to look at is what is the most selfless act? Does the fact that allowing the baby to be adopted frees up the mom to pursue her dreams make it selfish instead of selfless? Some people might argue that it would. I would not, as it doesn't sound as if this couple had intentions of getting married and staying that way for the long term at the time the child was conceived. (Correct me if it seems I am wrong... I am just going by my impressions from reading your 3rd party description).
A child remaining with his/her biological parents is not NECESSARILY better off, just because they are with their biological parents, or one of their biological parents. If the home is not filled with love. If the parents view the pregnancy as a burden. If the child turns into a wedge between the mom/dad. If if if if....
However, there are soooo many couples out there that are stable, loving, and long for nothing more than a baby to share their love with, to complete their family, but are unable to conceive, or to maintain a viable pregnancy. In an adoption situation like that, the child would be unlikely to be viewed as unwanted, a burden, or have resentments build between the parents. Rather than a point of conflict or stress, the child would be a ray of light and sunshine, further bonding together a couple longing for a baby and a family right now. Their "one day when I am ready" is RIGHT NOW.
It is very touchy to discuss this, because almost anything could be construed as a jab at someone. A single parent. A divorced parent. Someone who kept their child and got married for that reason. And on and on... My comments, I hope, do not come across as criticizing anyone. Not my intention at all.
Anything can happen. Good or bad. Whether she keeps the child or makes it available for adoption. There is no right or wrong answer.
I do wonder sometimes, however, if easily obtainable abortions have created a backlash against moms who might choose adoption for their babies. That may sound weird... but I know some late teen/early 20 girls, and the feeling I have observed is that it is a moral stain to "give your child away". That it would be better to abort it. And I don't understand that. The same women will vehemently declare that they could "never give their baby away"... yet they contemplate abortion as the alternative "solution". Why is adoption so morally reprehensible to them, and abortion is not?
There is no shame in adoption. It is a selfless thing. And a blessing to the baby, who will be received by his/her adoptive parents with nothing but gratefulness and love.