even Grandma softened up around him

[Mnemon]

Mom- I wish you could have known him.
Child- Who?
Mom- Grandpa.
Child- Even Grandma softened up around him.

From the movie A Monster Calls.

Here's the definition of the phrasal verb soften up,

soften up [transitive]~ to make someone more likely to do what you want by being nice to them
Example: His strategy was to soften them up with compliments.

Macmillan English Dictionary for Advanced Learners © Macmillan Publishers Limited 2007 © A&C Black Publishers Ltd 2007.

As it can be seen, it's a transitive verb. The question is why it's being used like an intransitive one?

Does it work for you if I say,

Even Grandma was softened up around him.

?

Thanks.

 

[5jj]

It can be used intransitively.

 

[probus]

Many verbs can be both transitive and intransitive. "Soften up" is one of them.

As to your sentence, it is understandable but unnatural. It would be natural without the "was".