You mentioned consulting firm, so I'm assuming he was offered partnership?
Every firm is different, but the firms I am familiar with basically have you buy into the partnership. The equity is non-transferable, meaning you can re-assign it or sell it to anyone else without certain share of votes among partners (very unlikely, for obvious reasons).
On the other hand, the partners tend to not have a ton of liability. Depending on the type of services the firm provides, the worst that could happen would be the firm goes under, but clients don't tend to sue consultants... they are just giving advice and/or outsourcing some work for the client. DO NOT assume it's risk free. Make sure you understand the partnership agreement and what may come out of it.
3) "expected" or "required"? These are two different things.
4) Generally the company will finance the equity purchase through either salary or bonuses for a few years. Most people don't have hundreds of thousands of dollars in cash lying around. It's possible to get a bank personal loan or other unsecured debt, but it is tough to get enough cash (most have limits well under $100k).
5) Get in contact with a self-directed IRA provider, and also read carefully through the IRS rules for IRA's. My assumption is that you cannot do this, since your payouts are too similar to salary. I'd speak with an experienced accountant to understand things better. An IRA provider may let you put whatever you want in there, but years later the IRS could come around and "disallow" the contributions and cause a real headache.
6) It's no different than 5 dudes (and/or ladies) getting together and opening a lemonade stand. Of course, your business will be more complex, but you basically put up equity and own a business together. Sort of like getting married, so just know that any real dispute will have to go to court, so make sure the partnership agreement is well written, and even more importantly, make sure you really trust the other partners on a personal level (for a small firm, especially).