He/she will need a letter of invitation from you and it will help if you state you will be responsible for any medical bills and extra expenses that may occur, the British Embassy in Qatar will check you on the British electoral roll to make sure you have a valid address, they may also ask for a bank statement from you to show you can support the person if necessary.
He/she will need a letter from their employer to prove they are employed and will return to Qatar, proof of how much money they will bring with them i.e. bank statement.
How do I know this, we had a lot of trouble getting a visit visa for my bro-in-law who worked in Dubai and wanted to come and visit, they refused and we had to appeal but won the appeal four months down the line, at the time he had an Indian passport but as he hails from Goa he now has Portuguese nationality and doesnt have that problem anymore.
It was quite strange, we filled in the appeal forms and sent them of in Jan and were given a June appeal date then one day in March I got a phone call from the Home Office asking me who our lawyer was as I had not put one down on the form I told whoever that I did not need a lawyer I was telling the truth my husband is reg blind and not allowed to travel so he wanted his brother to come and visit and quite capable of standing there and speaking up for myself thank you very much, in fact I went "into one" and had a polite rant - thank you madam said the Home Office man, next day had another call - but it was April Fool's day-so Iwas a bit wary - they said go tell your bro-in-law to go back to the consulate in Dubai and collect his visa !two days later we met him at Heathrow.