A friend has been overstaying in the US for 8 months, he has a valid tourist visa. Will he be able to go to Guam and get back to LA? Do you have to go through customs and immigration? Thank you!
He will most likely be able to make it to Guam - there are generally no passport checks, although legally he is still required to carry his passport.
However once he is in Guam, he's in for a world of pain.
Non-US citizens do require a passport to depart Guam, and he WILL be processed by CBP before departing Guam back to the US mainland. At that point his overstay in the US will be detected, and he will very likely be detained for deportation.
Note that despite your comment, he does NOT have a "valid" tourist visa. Once he overstayed any period of time (and especially after overstaying more than 180 days) his visa is no longer valid. Having overstayed more than 180 days, he will also be subject to a minimum 3 year ban when he does leave the country - or likely more if he is deported.
(There are a small number of exceptions to the above, such as if the person is below 18 years old)