Hi there, and welcome to NRaas. Yes, you are in the right place (sort of, will explain below).

I decided to invite her boyfriend over, who she met at the University, and ask him to move in.
Here's the first problem. When you invite a sim from a foreign world over (Uni World counts as foreign), it's a scaled down data representation of the sim who arrives for a visit. The real full-fledged sim is back in their homeworld. What we are meant to do is interact with this visiting sim if we like, up to a point, and any changes such as relationship scores and things that happen along the way are held pending in the TravelDB file until that sim is simulated back in their own homeworld again, then things just kind of catch up with them.

Asking them to move in, or forcing them to using MasterController (which you don't have), while they are in that less than a full sim state is not a good idea. If not enough data really comes over with the visiting sim, this will lead to the possibility of sim and game save corruption further down the road. Doesn't happen every time, but it certainly increases the risk astronomically.

To get a sim whose foreign world is elsewhere to move in properly:
-- Get them to join your household while your sim is in their world and thus bring them home with you.
-- When the sim is NOT visiting, use MC's Add Sim > Homeworld > (filter to their homeworld) to force them into your homeworld as a full resident in one step, adding them to your own household, that of another, or one of your own.
-- If you must work with sims you have called over to visit, have four mods in place that bring more data with the visiting sims and help to prevent the above-mentioned corruption. Those would be Overwatch, ErrorTrap, Traveler, and RelationshipPanel. You are missing Traveler. This still isn't the safest of the methods, but it's a step up from before.

...when I use the Passion mod, I get a script error for the boyfriend
I'm afraid it's not really our place to troubleshoot the mods of others. Yes, ErrorTrap is catching the error but it's Passion that is throwing it. Having said that though, looks to me like the mod is unable to get the sim to spin into what I assume would be their naked outfit as the mod calls for them to, or it could be sleepwear or swimwear, depending on how you have that mod's options set. Foreign sims do not arrive with all of their outfits, usually they have one or two and others are only generated as needed -- and they may not look as one might have expected back home. To check on just this, I would probably have that sim do something else that requires them being naked (like take a bath/shower) and during that see if their naked outfit can then be adjusted in MC's version of CAS/Plan Outfit (that is, MC > Stylist), or check on the other more standard outfits accordingly. That require actually having MC, though.

A <a class="wiki_link" href="/DebugEnabler">DebugEnabler</a> command, Fix Invisible Sims, can also help fix invalid clothing assignments even if the sim isn't actually "invisible." But that requires having the DE mod.

If I am misinterpreting the error though, it would really be up to Passion's mod developer to explain it further. But it is an outfit selection error that ErrorTrap is catching and the sim having been foreign and brought into your homeworld as a resident with not enough data almost has to be a factor here.