I don’t think she is being logged out if you say the test actually gets marked. It sounds like the time just ran out before she was able to complete the paper. Could you let us know the tests that timed out and we’ll investigate further.
The best thing would be to use the Question Shuffle feature to practice doing questions. Then once her speed is up start to do full tests.
By the way, credits are only taken the first time a test is assigned. You can assign the same test multiple times after that with no charge.
Will wait to hear back from you.