In my most respectful opinion, I think you are approaching this the wrong way. By focusing on what's hot right now, you are overlooking way too many factors, such as your market, risk, etc. Small video rental stores and computer stores were hot at one time, but depending on when you got into it you could have made a fortune or lost a bundle. Businesses don't just happen overnight, but a fad can dwindle that quickly. The key is not what's hot today, it's what's hot tomorrow.
My advice would be for her to use her market research skills to discover a market and then provide a product or service to meet the demand.
There's many questions to be answered, and I suggest talking to someone from the SBA (Small Business Administration) for advice. They have a pool of retired execs and entrepreneurs that will offer advice for free, and will be much more helpful than what you'll get on an internet message board.
Here's 4 to start with:
Are you going to participate in the day-to-day operations of a business? If so, she should concentrate on something she likes to do and knows alot about. I can't imagine taking great financial risk, dealing with legal issues, stressing your marriage and family, sacrificing vacations, working long hours, dealing with customers, etc. for something you don't like to do. Also, a business can and probably will fail if the owner doesn't know what they are doing, so going into something that you know alot about definitely helps. If you're not going to be involved, then be prepared for the additional costs to hire expertise and reliability.
Is it going to be a local and/or internet business? It sounds like you are talking about a local business. If that's the case, take a look at your locale to research the factors that can make for a successful business. Zoning regulations, local business laws, demographics, income, economy, tastes, real estate costs, employment pool, and many more local factors come into play. Look around locally to see what may or may not work before you go on the internet or hear the marketing spiels of the various chain stores. If you're going the internet route, the issues are how well you understand the technology, the costs and risks of it, and the fact that the internet is a Wild West when it comes to commerce.
How much risk are you willing to take? I'm talking financial, legal, personal, etc. Higher risk tends to have higher profit. But it's important to assess your risks since more than half of businesses fail within their first 5 years.
How much capital do you have access to? No use talking about a high overhead, high entry cost business if you don't have the capital to do it. No need to start on a shoestring if you don't have to.
Once you get an idea of the business you want to partake in, develop a business plan. A business plan is not only helpful for obtaining capital and outlining your business, it will help determine feasability of your endeavor. You may find that you have a winner, or it's time to toss it into the scrap heap and start again.