Unless I am mistaken and have made a grave error for the last three years, a cash ISA is treated just like any savings in the US. It's a stocks-and-shares ISA that US expats should avoid because they are treated as a PFIC.
She should report the interest in her return, but it is treated like normal interest income in the US.
Also, if she reaches a combined threshold of $10,000 (this is adding up her ISA and her current account, and any other assets she is signatory on, such as joint accounts with you, if there are any) at any point in a calendar year, even for a minute, she must also report all of her foreign assets by filing FBAR the following year. For example, if on March 12, 2018, she reached a total $10,000 in her UK accounts, by October of 2019 she must file FBAR for 2018, even if her balance went below $10,000 on March 13, 2018. For subsequent years, any time she has had at least $10,000 in foreign assets, she must file again in the following year.
If she's only going to hit $10,000 for the first time in the next few weeks, she has until next October to file her first FBAR. It's really easy and straightforward to do. It's just annoying that we have to do it.