Upstairs? How long are the cables? I know there are limits with runs of analog cables like VGA. IMO you should make sure the cable length isn't causing any degradation before getting a new video card with two outputs only to discover it does the same thing.
You can run some pretty long lengths before you start getting problems. 50' HD15 cables aren't hard to come by. I spotted one product that recommended 50' HD15 cables
then converted the feed to a wireless signal. It's used in presentation and signage.
You're right though, he does need to test both lines before plopping any money down for a solution that may or may not work. If he's tested each line individually without problems. Then the problem might be the split, it's dropping the signal too much. Either get a VGA amplifier like the ones that can be found on Amazon or get a dual header video card. A two port VGA amp I found on Amazon from StarTech is selling for $37 and change. I don't know how well it works, never wanted to amplify the monitor signal before. But $40 is a worthwhile shot to save paying $300 for a new card.