Reading between the other mum's lines, here's my guess
She thinks your DS is a nice lad but perhaps rather needy (lost his dad; anxious to please GF,. latched onto GF's dad and spending a lot of time at their house. ).
Her daughter is in college; training,maybe exams coming up, so she needs to spend less time with BF and more time studying/widening her interests/skills/ furthering her own independent future. She has big plans, travel etc; and she's very young.
I note. zero mention of DS's skills, plans, future.
I suspect Mum is worried that when DD finishes college, instead of launching off into the world, she will feel, or be, anchored by and to DS. That they are both far too young to settle. That her DD might miss the boat and be marooned on the home shore with homeloving unambitious grieving/bruised/needy DS.
So she wants them both to take a step back and give each other some space and for your son NOT to hold back her daughter next year (or whenever she leaves college)
Other |Mum has tried , in the nicest possible way, to engage your recognition and support but you are just not quite getting the message. (Understandable that you're still adrift and grieving too).
Of course your bereaved son has attached his need and affections to this kind sweet girl, and he thinks she's the love of his life. But as an experienced adult, you must know, the first little beginner romance is not the be-all and end-all. What your DS needs is to focus on his own plans for an independent future, a career, hobbies, skills; social circle and NOT put all his eggs in one basket labelled GF.
Because if he does that, then in a little while he might feel desperately hurt and abandoned when she takes off to travel, work abroad, leave him behind etc.
He's already lost one dear love and support, his dad. Don't let him risk replaying that same scenario all over again if he parts from the GF and the second fatherfigure/family he's found that their house. . Help him to separate, stand on his own feet. For his sake.