Everyone else has covered a lot of the low-hanging fruit here, from asking "why is your party splitting?" to making suggestions of varying potential applicability, but here's one point I think is worth making and one suggestion no one has made yet.
First,
Classical Alignments Shouldn't be Considered Concrete Rules
It sounds like your players are putting a lot of weight behind that "Lawful" label, which is a common bar people run into that prevents them from roleplaying realistic characters. In real life, no one is 100%-always-no-exception on one end of the alignment spectrum. Even the most rigid, by-the-book people often have particular exceptions in life that they don't personally respect.
We don't know the specifics from your brief example, but hypothetically if the heist was of an ancient relic from a local lord who, in the past, hired mercenaries to collect by force, sure he might have gotten his hands on that "legally" (he's a lord, after all, he makes the laws!) but that doesn't mean that the player characters would necessarily respect that ownership as legal or regard taking it as violating a meaningful part of the social contract.
Alignment is not meant to be a hard rule, but rather guidelines for your roleplaying. If you have a character who is feeling boxed in by their alignment to the point where they're not participating in an adventure in ways that would actually make sense for those characters, they should not feel constrained by it.
Second,
What to do When You Must Have an Extended Party Split: The Extra
Sometimes the party just has to split for whatever reason. Maybe the reason is realism (we're not all joined at the hip all the time) or just a desire to highlight a particular quibble a certain character has ("Not a heist! I hate heists. Count me out.").
When this happens, what has worked for me brilliantly in the past is to have the party grab an extra head to get them back to the party size they're used to, Ocean's Eleven-style. "Well, the bard is out, and he's our lock-pick. We better hire someone to cover that base for this job!" And then have the player of the character that's sitting out play the fill-in for that session. It can be a fun excuse to ham it up as a extra for a session and, you never know, they might prefer the fill-in character and decide to make the swap permanent, preventing this from happening again.