You can post a pic right here in your message. If by "trimmed", you mean he cut a lot of branches that were 3-4 inches in diameter, you might have a problem. If by "trimmed" you mean he cut a few branches that were hanging down or growing over the house, then there likely will not be a problem. If by "trimmed" you mean he cut all the branches back by 10 feet, then you may have to trade in the in-law. If it is his/her tree, there is not much you can do. If he did it to "help" his in-laws out, then he should have his chain saw banned form the property.

Post a pic and we can give advice. Otherwise, we will need to do a mind meld so we can see the image you have in your brain.

I'll bet it'll survive.
It may not be aesthetically-pleasing, but it has been a fairly common practice in old non-productive pecan orchards to shear those senescent trees back pretty severely. They respond by putting out some aggressive re-growth and are stimulated to come back into production. They're not pretty, but they make nuts!
This is usually done while interplanting younger, more productive cultivars, with plans to remove the old trees once the newer trees come into production - but it's a way to maintain some cash flow while waiting for the replacements to reach bearing age.

'Production' trees often have a much different appearance from 'landscape' specimens. Many folks might not like the appearance of most fruiting trees pruned to maximize production - gracing their front yard. If this tree's main purpose was to shade the family home, it may not be ideal, but not worth starting a civil war over.

I'd not thought about 'dehorning' pecans in a number of years - the linked article below indicates that it's less desirable in bringing production back than just removing some trees in an aged orchard.

Here is a link that might be useful: Pruning Mature Pecan

Given the habits of my Southern brethren, I truly hope he didn't top the pecan.