Should we invest in a company right before retirement?

My husband has been offered a partnership in a construction co that he has worked at for the past 21 years. What questions should we ask before jumping in? The owner is 60, looking to retire in 5 years. There are 2 other employees being offered the same deal.

This has taken us by surprise. This is a commercial construction company in its 30th year. He had 2 other people in the office buy in a few years ago for bonding purposes, but now has offered 3 others the same. My husband is the field supervisor and handles the men very well, but we are not business savvy. My husband is 51 years old, so he would like to retire in 10 years. We would have to borrow the money to invest. All our major money is in either Roth, 401K or mutual funds for retirement. What should we ask for a feeling of comfort? Construction has been slow, but that business does go up and down.