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The criteria to be granted a 6 month visitor's visa is that they must intend to leave at the end of his visit, has enough money to support himself during the length of his stay without out working or needing public funds. He must not undertake paid or unpaid employment.
You can help by writing a letter of invitation corroborating his statement as to why he is visiting the UK, ensuring that he has accommodation whilst he is here (ie. by either staying with you or arranging for a place to stay), funds in his account to show that he can support himself without the need to work.
They may be suspicious that he only intends to come here either to marry your daughter and then apply for leave to remain (which he would not be permitted to do) so he should make it clear that he has a life in Tunisia to return it. Evidence of his full time employment and perhaps a letter from his employer confirming that he is leaving or a visit only and that he is expected to return to Tunisia to work. Evidence of his accommodation will help if it shows that this is continuing (eg tenancy agreement or statement of landlord) and that he has somewhere to return to.
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