These international foundation courses can be a bit controversial. The original idea was that they should provide an entry route for bright international students whose prior study did not provide them with the foundations necessary for degree level study in the UK, largely because of the curriculum content, level, language or length of time they spent in education.
However, they increasingly seem to be used as a second chance for any international students who have simply not achieved the necessary entry requirements for any reason and who can pay the fees. Foundation degrees, and in particular ones specifically for international students, are low risk for the universities. They attract international students (and their fees) and often involve minimal or no teaching by actual academics from the university itself but if the students don't meet the required standards during the Foundation year then they can just let them go while keeping the money the students have paid for the year in the hope / expectation that it will get them onto the degree course.
The Bath model essentially involves the students doing the rough equivalent of 3 A Levels in a year, with acceptance onto the degree course being contingent on the students getting grades that are approximately equivalent to the actual A Levels required for normal first year entry. The benefit of the foundation year is the guaranteed entry to the undergraduate degree if you get the necessary grades. If a DC has had a normal educational journey in their home country and has not come out of it with the grades necessary for normal entry, you need to consider whether anything would be different with the extra year, i.e., if they have always got Bs, what makes you believe they will magically get As in this year? Otherwise, you are spending a lot of money on a one year qualification that leads to not much at all and there are surely cheaper ways to retake or add qualifications.
Other models focus more on just doing the degree subject and may be more suitable for people with very specific interests and talents that mean their prior attainment is not indicative of their ability to handle degree level work in that subject.
As with all Foundation degrees / courses (excluding things like Foundation years in subjects like Art which have a somewhat different purpose) the important thing to consider is this: if you couldn't make it once, what makes you believe you will make it with a second chance? There a many reasons why someone might not have made it once; inadequate teaching, changed perspective as you get older or discover more about particular subjects, illness, interrupted education, and so on; but absent any specific thing you can point to as causing the issue, the DC (and the people paying their fees) need to be very honest with themselves about whether they have the academic ability, interest and will to achieve to a level they haven't so far, and to achieve it in a relatively short space of time, on the foundation degree and (if they make it) subsequent undergraduate degree.
Universities are not always as open and honest as they could be about this, because they want students (and in particular they want international students and, in some cases, are willing to engage in quite underhand practices to get them). They let people believe that they are offering these students a wonderful chance, and in some respects they are, but when this chance is contingent on the students achieving something they have hitherto not been able to achieve despite favourable circumstances and thus lack any real evidence that they might ever achieve, there may be more appropriate (and cheaper) courses of action for them.
This would be my concern about this route for your DC where the sticking point appears to be their ability to do well enough in AP exams, as it calls into question their ability to achieve at the necessary level in higher level courses. If they had a bit of a dodgy GPA because they struggled with certain required subjects that were unrelated to their proposed subject of study, that might be a different thing.