Hi again, computer problems!
Right then, if he sells his house in 10 years time he will face a taxable gain of £50,000. Current CGT rates are 18% for basic rate taxpayers and 28% for higher rate taxpayers; these could have changed by the the time he sells.
However, a simple route is for your friend is to live in the house for at least 6 months prior to selling it; it will become his primary residence and there is no CGT to pay on any profit from your primary residence. My friend has a portfolio of around 20 buy to let houses, and used to renovate and sell several per year, and used this method very well. I even supsect that he never actually lived in the houses just got his post sent there and registered on the electoral roll (Please note that I'm not suggesting you do this as it is illegal).
I'm not sure what your point is in your second paragraph to be honest but you would not be able to get a 100% mortgage anyway and you would have to pay interest on your loan. Again, why not live in it for 6 months, give it a splah of paint and flog it for what you can and have no CGT liability on any profit?
Hope this helps?