If it is a UA bump certificate it should be 4 $75 certificates, which adds some flexibility. These certificates -don't- stop you from getting miles. It isn't really clear--do you need to travel with this person, or can you split up? That adds some possibilities. If you don't, I'd get the add'l 5000 miles on UA somehow, book my friend on the free ticket, use the $300 for me and pay the difference and get the miles on UA