I am planning on getting a mortgage (first-time buyer) later this year. If I pay my friend's IVA debt will this affect me negatively in the future i.e credit score, mortgage?
No --- as far as the world at large are concerned, if they ever need to know (which they don't), you simply gifted a sum of money to a friend. In terms of credit worthiness or financial history you will not be linked to the IVA in any way.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014