What you paid for it and what you put into it is not relevant anymore.
What matters is the value today; the amount outstanding today and the terms of the mortgage.
It sounds as if you both want to reach a fair deal, so your friend would not exploit the legal difficulty you might have in getting out of the deal.
So, it were not a tracker mortgage, the calculation would be very simple:
Value: €250,000
Mortgage:€210,000
Value of asset: €40,000
Your share: €20,000
But the tracker makes this much more complicated.
If you were trying to maximise your share you would argue as follows:
You have a mortgage of €105,000 @ 1% over the remaining 15years. The repayments on this are €628 per month.
The repayments on €88,000 @3.5% over 15 years are also €628 per month.
So if she can take over your tracker, she is getting something worth up to €17,000.
It will only be worth this if she keeps the mortgage for the full 15 years and if the gap between the tracker rate and the non-tracker rate remains at 2.5%.
If she loses her tracker when remortgaging, then your tracker has no value at all.
So, she should apply to your lender to take over your part of the mortgage. If they approve and don't try to push up the mortgage rate, then you could do the above calculation and see what result you come up with.
They may refuse to allow her take over the mortgage at all, or they may give her a new mortgage and take the tracker rate from her.
If they refuse to give her a mortgage at all, I don't think you should force a sale. If they insist on taking the tracker rate from her as a condition of taking over the mortgage, then you should retain your share of the property. She should pay the full mortgage. You would own half the equity i.e. as she pays down the capital, the value of your 50% increases. If the property falls in value, your share will fall in value accordingly.
At some stage in the future, as the mortgage capital is paid down, the value of keeping the tracker will reduce, and you may choose to go your separate ways at that time.
Whatever you agree, make sure to put it in writing.
But first of all, she should approach the lender and see what their attitude is.
The key thing here is that you have a very valuable investment. A cheap tracker on a property in positive equity You probably should not give it up unless there is a very good reason to do so.
Brendan