Adding dog into photo and fixing shadow

[Jfurlong]

This was an end of life shoot for the darker Doodle on the left so I can't recreate the shoot.   I've been attempting to add this pup into this group shot.  I had to take the Boston's photo the next day cause she was completely not cooperating with her siblings and of course, the shadows are different. 

Is it possible to salvage this to make the pup look more integrated into the shot and to fix the shadow on the Boston?

I can send the original layered .psd if needed or even the processed individual files.   

Thanks for any help in advance!

-Jill

Edited May 12, 2017 by Jfurlong
forgot to sign the post :)

[Damien Symonds]

Gee ... you kinda need to flip the dog horizontally, don't you?  So that the shadows go the right way?

[Jfurlong]

Yeah.  Kinda.  But wouldn't that be apparent that their pup is basically backwards?

 

[Damien Symonds]

Yes, I fear so.

What about flipping the rest of the photo?  The other dogs don't have as many distinguishing marks.