Amending Filing Status

My daughter and son-in-law decided to file MFS due to her student loans. SIL's preparer filed him as single in error. So that has to be fixed. I went to do her taxes today, and now that I am looking at their numbers, it may be better for them to file married joint. Here are my questions:

 

Can SIL's tax preparer amend his return to be married filing joint and add daughter's information?

 

Should I file an extension for daughter, since technically she will be filing late? If I remember correctly, amendments can't be filed until after April 15th. Is that still the case?

 

I appreciate the help!

Lisa