Out of status from last 15 years, getting married to citizen now. what are the chances/procedure of getting him GC?

No one could produce any assurances without actually analyzing, preparing and submitting all requisite documents in that AOS case.

Your friend and his girlfriend will definitely need to consult an immigration attorney. It may be possible to Adjust Status if the person is merely in "Overstay". But it is important to seek proper legal advice before filing.

if he entered the US legally then the marriage may give him a way to get his GC. as to the rest a full detailed consultation is needed.

There are a series of questions that would need to be answered before determining what the options are. They depend on the girlfriend's legal status, how he entered the U.S., if there are any other complicating issues, and more. An immigration lawyer would be able to address these concerns. I strongly recommend he consult with a qualified immigration lawyer prior to proceeding.

Hello, Your "friend" will only be able to apply for and obtain a marriage-based "green card" if he has previously been admitted into the US legally, i.e. with a visa and passport (and can prove it.) 1. The "how long" will directly depend on the specific facts and circumstances, as well as the reputation and experience of the immigration lawyer he will hire for this operation. Thus, assuming the entire "adjustment of status" package is accurately and correctly filed with USCIS the first time and there are no mistakes or omissions which could delay the case, in San Diego "friend" (and spouse) should be scheduled for the I-485 "adjustment of status" ("AOS") interview within 2-4 months from filing the "one-step" I-130 and I-485 applications, including the I-864 Affidavit of Support which the US citizen spouse will need to file on his behalf. Within 2-4 weeks from filing "friend" will be scheduled for fingerprints in San Marcos, and within 2 months after that receive his "work permit". (Sometimes the "interview notice" at USCIS arrives faster than the approval of the work permit.. 2. Need to examine the facts and circumstances in detail to be able to meaningfully answer that. Some example of "complications" include, but are not limited to, proving the marriage was entered into in "good faith" for love and not with the primary goal of procuring a green card; credibility issues the USC spouse could face; proving lawful admission; and etc. 3a. To apply will need the proof of US citizenship of the spouse, marriage certificate, proof of lawful admission into the USA; foreign birth certificate and translation; medical report; and etc. 3b. To the Interview with USCIS will need to bring all documents proving life together as a husband and wife since date of marriage (and way before in this case, will also help..), commingling of assets, etc. Once the time for the interview comes and they receive an Interview Notice letter from USCIS, that Form will list (almost) all the type of documentary proof to be brought to the interview.