Ronnie's a bad influence on her. Keep him away.
Can we use an apostrophe and "S" to say is at the end of a name to say somebody is when writing informally?
Ronnie's a bad influence on her. Keep him away.
Can we use an apostrophe and "S" to say is at the end of a name to say somebody is when writing informally?
Yes, this is correct. It's not required, but it's okay to contract names with "is" when there's a direct object, like here. We wouldn't say things like
Who's going to set the table? Ronnie's.
In this case, we'd never contract. Formally, it's better to avoid contraction, but in any case where contraction is okay, this is the way it would be done:
Who's going to set the table? Ronnie is.
And your case is correct as well:
Ronnie's a bad influence on her. Keep him away.
For the case where the name ends with an "s" sound, the proper way would be:
James' a bad influence on her. Keep him away.
Others argue that it might be better to simply not use any apostrophe, ie. "James a bad influence". In either case, not contracting at all is acceptable.