need to convert US recipe calling for tomato sauce to equivilant in UK
Tell your friend to buy around a 450g jar of passata. This is finely crushed, sieved tomatoes. It comes plain, or most supermarkets carry versions with onions, basil etc.

Tomato puree is an entirely different thing and I wouldn't go down that route.

There are also various pre-made tomato sauce jars available from the likes of brands like Dolmio that have various flavour combinations. Depending on the ingredients in the tomato sauce you use, one of those may be more appropriate. They tend to be thicker and more heavily seasoned and flavoured than straightforward passata.