If request was send and wasn't ignored by choosing that option next to friend invite- it's still on the list of friends requests- but from what I know it will only show up again when another person will send such request and that pop-up will show- then you will have access to whole list of players who send you invite and you didn't react to.

So, ask someone else (or use alt for that) to send friend request to your friend when he will be online- and he should have your request on list there too and he can accept then.