My Daughter Was Married In Tunisia Her Husband Is Now Living In England But She Would Like To Know If The Marriage Is Legal In The Uk

As long as it is a valid marriage,

Assuming your son in law obtained his UK Visa due to marriage then the answer is Yes, the marriage is legally recognised.

She has to regrister her marriage in the UK with the original documents from the Municipality where she got married from in Tunisia. If she approaches the Embasey in London they can help, or indeed Citizens Advice oor any lawyer