You will lose the tracker.
Effectively you are remortgaging - your new mortgage will clear the old mortgage. You could stick with the current bank or move to another lender for a preferential rate.
There are three problems with your scenario, though:
1. It's extremely unusual to have two mortgages against the property. The only case is in the event of a top-up. And you could not do this, since the top-up must be in the same names as the original mortgage (which has you and your friend on it).
2. As other posters have already pointed out, you should consider the fact that a bank might not lend you the funds you need to take over the entire mortgage.
3. You say your friend wants to "sell his half of the property" - will this mean he can clear his half of the mortgage, too? For example, if property is worth 100,000 and mortgage is 50,000, you will buy out your friend for 50,000 which is half the value of the house. Presumably you don't have 50,000 in cash since you said you need to get a mortgage for it? This means the mortgage will increase to 100,000 (original 50,000 plus the new 50,000). A bank won't lend 100% of the value of the house, so you'll have to come up with at 8% (for a 92% LTV) from your own pocket - not a loan.
Obviously this is not a problem if the value of the property far exceeds the mortgage (ie by more than 50%).