I know of a person that is livivg in the uk with a passport with false information on it.so basically he's an illegal immigrant.hes been livibg here for about 20years and has been married to a british citizen but is going through a divource.will he be able to saty in the uk?

What do you mean false information on it? Are you sure it's really a fake passport? If he's living and working in the UK he must have a social insurance number (or the equivalent there).

Why are you so sure he's not a legal citizen is basically what I'm getting at.

Well apart from the above, you need to explain, does he have a foreign passport with a UK visa or a UK passport that you believe has errors in it. If he has a UK passport he must be a citizen so no he won't be deported.

If he has a foreign passport and was granted permanent residence due to the marriage but is not yet a citizen then most likely no also. A permanent resident is just waiting until enough time passes that they can apply to become a citizen, he may have to file papers because of the divorce to request that his status remain the same or re-apply as an immigrant worker etc but they won't immediately throw him out because of a divorce if the laws are similar to those in Canada.