A Will has no effect until a person dies, meaning it is not possible to challenge a Will until that time.
If you are concerned about whether a Will is valid, it may be worth speaking to them to understand their decision and check whether they are happy with it. If they are not, they may wish to update their Will.
If they lack the capacity to do that (maybe because they have dementia), please contact us about the possibility of applying for a Statutory Will.