OP, as already pointed out, if the people you know have either EU settled or pre-settled status, or a 'certificate of application', an EU Settlement Scheme Family Permit, a Frontier Worker Visa, or are an S2 Healthcare Visitor, they have the entitlement to use their national ID card to enter the UK. They have this right for life (if they have indefinite leave to remain) or for as long as their visa/permit is valid for.
HOWEVER.
Before they go, get them to check that their card has a biometric chip. You don't need a card with a chip to travel right now, but this rule is likely to change in the not too distant future, so worth checking on gov.uk before they leave.
Also, there have been instances of carriers (airlines, ferry companies) not understanding the rules and refusing to onboard people with ID cards. They should know, but occasionally it goes wrong.
What your friend should do, is log on to their UK Visas and Immigration (UKVI) account by going to the View and Prove service (https://www.gov.uk/view-prove-immigration-status). This will give them access to their online immigration status (otherwise known as an eVisa). They should screenshot this before they go and have it to hand.
If the carrier won't accept a screenshot, you can also go to the same service and generate a 'share code', which you give to the airline. They can enter that share code and see the immigration status for themselves by going to this page https://www.gov.uk/check-immigration-status. This will prove to the airline that you have the right to enter the UK as part of the Withdrawal Agreement, and with that, the right to use your ID card to do so.
Final tip: If your friend has permission to live in the UK under the Withdrawal Agreement (EU settled or pre-settled status), they would have entered details of their documents (passport, ID card, or both) as part of their application. Since they did that application (probably in around 2020-22) if they have renewed any ID document (because it expired) they need to go back to their UKVI account and update their document details – because if their new document details don't match the original ones stored in their UKVI account, it will cause problems.
If after all this they are still refused, they are entitled to compensation, as well as reimbursement, re-routing or rebooking. Hope this helps :)