As for Downy himself he's a little outnumbered at the moment so any potential ally no matter who it may be would be better than none, and if he really does have the Seals then he wouldn't have to worry about any type of betrayal from her. (For now at least.)
Also I'd like to add that the contract he has with Imnity at the moment is actually similar in nature to that between a Servant and a Master (hence why Imnity calls him Master.) so even if he doesn't have one he could establish a connection of sorts if she accepted his offer. (He's got a pretty darn big supply of magical energy at the moment as well since he hasn't really been able to use it at all against his current foes, thanks Elf lol.)
Of course this is completely up to how Daiki wants to play it, depending on what he thinks the Seals could have already appeared for Downy. (He just hasn't noticed them yet what with being busy with all the stuff going on.)