First would be to cut out all the old wood, but I would wait until the new growth starts, just so you can see where it is. The never-blooming thing is weird; is it in a shady spot? Are they sure it's a forsythia? {not to be snarky - just want to be sure).

Where are you, or actually, where are your in-laws? In my area a non-blooming forsythia probably doesn't have flower buds that are hardy enough. So when I brought a cutting of my grandmother's central NY forsythia bush here to NH, it only bloomed where it had been buried in snow because the flower buds were killed by cold. Years when there was little snow when it first got really cold, there were no flowers at all.