We know a young adult 4-5 yrs out in the work world. Recently went through many rounds of interviews with a company and a offer is in progress. Today it was announced the company was purchased by a well known online company. What advise would you give?
Added to the fact that the person is very happy with their present job.
Wait and see what happens. Keep his options open. They may freeze hiring. They may consolidate his area and fire a lot of people, they may not.
If he does receive an offer, I think he needs to have a very frank discussion with the person who is hiring him about what is likely to happen in the near future, bearing in mind that that person might not really know and may be harboring unrealistic hopes or putting his/her resume out there even as we speak.
He should ask to see an organizational chart of the post-takeover structure if they have released one to the employees. Check out which company the executives in each area will be from.
He needs to get some idea of why the other company wants to buy this one: what is the desired asset? Read the business press. It is probably reasonable to assume that the people most directly responsible for keeping the desired asset functioning are safer than most others. (That is, if you are acquired for your network of retail outlets, they aren’t going to start by firing the people who staff them. If you are acquired for your software, they aren’t going to start by firing all of the developers. Not yet, anyway.) Except the top brass, who will have negotiated a good deal for themselves, even if that means throwing half their staff under the bus. A lot depends on what he does. All kinds of support functions can be on the chopping block. It also depends whether the acquiring company needs to wring more money out of the company immediately in order to pay for the purchase/takeover. Oh, and another factor: would any of the company’s functions be moving to another location? Financial functions are often prime candidates for consolidation in one location. If so, they will want to keep moving expenses for employees to a minimum and you can figure that almost all of the people in that function are history. Bearing in mind, of course, that which location remains open can depend largely upon where the CFO would rather live.
Can you tell we have BTDT when it comes to these situations? 
Advice: Keep your options open. Don’t burn bridges. Wait as long as possible for the dust to settle.
I would not take a new job with a company that has bee acquired by another UNLESS I knew exactly what their business plan was.
He likes his current job…then maybe this is a good reason to stay there for the time being.
An offer may not materialize. My advice is to wait until the offer comes through. The terms might not look as attractive as he hoped for.