Maliko1 bhai. Your friend's idea for REVO as a Yellow Cab is definately a very worthwhile and workable idea. This car is perfect for a cab. It has a lot of room for luggage even after the CNG cylender installed, as well as passenger space. He can get Revo 80 (800cc) with CNG installed from Factory for only 309,000 Rupees.
While Mehran, which is not only has smaller passenger room and smaller trunk/boot space for luggage, looses even more space when you put a CNG cylinder in its back. And Mehran costs around 350,000 Without CNG and with CNG probably close to 400,000 Rupees, including the "ON" or dealerships's charge that many dealers are still charging on most cars.
The biggest BONUS for getting the REVO as TAXI.
It is definately a better deal, better looks. Most TAXI passenger prefer a bigger car with New Latest Looks, even if they are going in a taxi. Most people just hate sitting in a Mehran or FX, but only because those two cars look small and passengers think those will cost less for a trip, compared to a bigger car with TRUNK, in many cases it is true. A small Mehran guy can charge you less and get more passengers, because of Mehran running on CNG can run very cheap.
But now, REVO is in the scene in Pakistan, and REVO as a TAXI will not only be cheaper to operate for Taxi, it will not need as much maintenance, and if it did needed some parts, the good news is, all the parts for REVO are very cheap, as cheap as Mehran's and many are infact EVEN CHEAPER than Mehran, because Adam Motor's sister company, Omar Jibran, even supplies many parts for even Mehran to Pak Suzuki.
So, REVO is good both as a personal car as well as a TAXI.

So, tell your friend to Go for IT. This car is the best deal, and best deals do not come often, specially in Pakistan. This is the good time to buy it. Who knows, a year from now, when there probably WILL NOT BE MEHRAN, as Suzuki is thinking about stopping to make it, REVO prices may even GO UP, due to no competition left. So now is the best time to book your BRAND NEW REVO for only 279,000 Rupees.