"He is worried more about legal action rather than the moral one because he feels like they have had it easy and not showed J any compassion."
It is very difficult indeed to successfully make a claim of undue influence in order to contest a will. B and C would need to prove that there is no other reasonable explanation for J changing her will other than undue influence from A.
But, even so, it would be best to mention this concern to the solicitor who is drafting the will. They will likely give J and/or A advice.
One other area where it may be possible for one of them to claim against the estate is if they are in real need or hardship. There is often an element of ill health or disability or using the money for a specific purpose involved in successful claims.
I'll give you a couple of examples where this happened.
In the case of Elena Nahajec who was 31 and hadn't seen her father for almost 20 years, both she and her half-brother were awarded amounts even though the father had specifically included a letter explaining why he had disinherited them.
The estate was worth about £250k. Elena, who worked part time and wanted to train as a veterinary nurse was awarded £30k. In a separate case her half-brother who was disabled had settled for £22k.
Nahajec v Fowle [2017] EW Misc 11 (CC)
In the case of "H Deceased", his entire estate, worth £554k, was left to his spouse who was then in a care home. The son was the executor.
The 50 year old daughter had a complex psychiatric disorder, considerable financial problems and she lived in substandard rented accommodation with two young children. She was awarded £139k
Re H (Deceased) [2020] EWHC 1134 (Fam)
In the case of Rochford v Rochford in the Peterborough County Court in 2021, the father had mostly disinherited his only daughter. Out of an estate worth £245k he only left £25k to his daughter and everything else to his sister (the daughter's aunt).
The daughter had a severe degenerative disease which prevented her from working. She was awarded a further £85k and also costs against her aunt. The costs are likely as much as the award that was made. These articles from the solicitors that represented the daughter:
https://todayswillsandprobate.co.uk/a-stark-warning-on-costs-in-contentious-probate-matters/
https://www.shoosmiths.com/insights/articles/the-adult-child-strikes-back-rochford-part-36-in-inheritance-act-claims
But if none of these circumstances apply, then the person is unlikely to be successful.
For example, in the case of Shepton v Seviour in the High Court in 2020 an adult daughter who was comfortably off made a claim on her father's estate which went to her stepmother (even though stepmother was leaving her an equal share on her own death). The judge said that her case was "absolutely hopeless" and gave her nothing.
https://www.freeths.co.uk/insights-events/legal-articles/2020/the-good-the-bad-and-the-hopeless-shapton-v-seviour-2020/
Or the case of the children of Anthony Shearer. When he died, his estate was worth around £2.2 million. He left everything to his second wife and nothing to his two daughters who were then in their late 30s.
Again, the court gave the daughters nothing:
Miles & Anor v Shearer [2021] EWHC 1000 (Ch)
That case even made it into the Daily Mail at the time:
https://www.dailymail.co.uk/news/article-9504515/Millionaire-banking-bosss-daughters-LOSE-court-battle-step-mother-7m-estate.html