This was something that I looked into last year(at least I think it was last year-they are going too quickly for me :) )
DS was/is on Azathioprine(different drug group to Humira but also a drug that suppresses the immune system) I did a lot of online research and came across several reports that said that current advice was for patients with a suppressed immune system NOT to take pro-biotics(there had been deaths in people with a really suppressed immune system). However I was also aware that many Crohn's sufferers did take pro-biotics and have felt positives rather than negatives.
I spoke to people who produced the pro biotics that son was on and they advised for him not to continue on them.
I was still scratching my head on this one and spoke to a friend who is a consultant (in a different field) who, however, prescribes vast amounts of immune suppressants at a far greater strength (think transplant strengths!) Her opinion was that the strength prescribed to a crohn's patient was not that big and that there should be no problem and that the issue came when the immune system was severely compromised(like in transplants/chemo etc)
I don't know the strength of Humira with the immune system (although I am aware that it is considered one of the 'big guns') so I really don't know. There are many on it with no problems BUT your GI really should know all the pros and cons.