I had this same scenario--and we were in the Magic Kingdom yesterday. My husband (has AP) was not with us and my parents (do not have AP and unable to pre-book FP+) went with us. I too wondered if this would work, so......I booked my FP+ for myself, my kids AND my husband. It worked! Even though his card (magic bands shipped today but did not have yet) was not activated for admission that day, my dad was able to use his card for FP+s reserved. It worked very well for us. Here's to hoping it wasn't a fluke, b/c I often go with others, as my husband isn't able to go nearly as often.