Personally, I think that anybody who just throws something perfectly good and reusable away is just lazy and selfish. I recycled, reused, etc., long before "green" was a pop culture adjective. The Adidas gym pants I'm wearing came from the pile of stuff of the abandoned rental property next to our house. (The Tshirt I'm wearing is a picture of Snoopy laying on a picture of the earth that reads, "Save Our Planet" underneath. My MIL got it for me from Delia's last year because I like Snoopy and because conservation is second nature to me, as I was raised that way. Even if she and my SIL got the buy one half off deal, I think $15 for one Tshirt is very expensive, especially since I wear it around the house and as a night shirt.)
I think donating them to a place like a women's shelter is a good idea. Also, crisis prengnacy centers might be good, too. Lutheran World Relief collects cloth diapers (the old-fashioned tri-fold ones) and sleepers and Onesie style Tshirts and receiving blankets. http://www.lwr.org/beinvolved/layettes.asp
If anything, offer to clean out the garage for her and donate it so she doesn't have to worry about it. Or, if you're planning on having kids, keep it for yourself. (Then pass it on when you're done.) Baby stuff is rather expensive and every little bit helps. (Most of my daughter's stuff is either second hand or baby/shower gifts. My MIL buys my daughter's new, but my mom is great and getting stuff cheap or free second hand from rummage sales, things people leave behind in the rental properties she manages. I also get stuff from my sister who very likely got it second hand and cheap as well. That is the best kind.)