If you mean the The common ash tree Fraxinus excelsior? it can grow to a height of 25-35m, with a spread of up 20m, which usually means the roots may reach 20m too? Its not just foundations its drains that may well be damaged and expensive to repair and any problems are usually not noticed till too late?
This is not usually a tree for a small garden and your neighbour may be right to have concerns. I would get an expert to advise you re exact type of tree and what to expect re growth, removal. They are quick growers and the longer you leave it the more possible problems and costs involved? I would suggest that if it has to come down you discuss the costs of removal and suitable replacement with your neighbour?

Ash is also prone to dropping branches, and it seeds liberally all over the garden. It makes wonderful firewood and burns well even green. You could replace it with a mountain ash perhaps, same leaves but pretty flowers and red berries, but still not near the house. Your neighbour is right to be concerned.

A possibility is to prune it every other year to keep its size down. A pruned Ash has a much thinner canopy and you can keep it to a decent height, preventing root spread, but am not a fan.

If the Ash tree is less than 45 feet away from your neighbour's house, then you either need to consider removing it before it affects their foundations, or have it pruned every other year to keep it smaller, as Kildermorie suggests.