What you need to do is book a MAN-BOS return ticket for your partner and a BOS-MAN return ticket for yourself. The price should be similar. Yes you won't fly the MAN-BOS part but oh well. Unlike hidden city ticketing which someone else suggested and is doable only without checked in luggage, there's not much problems with this.
I see the flight you mention by just searching on Kayak and if I search on Kayak for BOS-MAN return with flight out Nov 15 and return Nov 25 it's the same flight out as you wanted and it costs $738. Search Wednesdays for the return date with +-3 days (since you don't care when it is), perhaps you can squeeze it a few more dollars downwards but it's close enough I think.
Here is your partner's and here is your flight.