Mike,
Just for the sake of discussion, who is the consulting group? As mentioned earlier, the fact that they are talking about asp rather asp.net says a lot about what level of expertise they have. If your client wants your take on it, ask the consultant to give you the data on which they base their findings. You can say a lot of things, but the truth is - if the site is doing what the client needs and is flexible to expand to areas they envision, then that's what matters most. Yes, it's true that finding hosts for asp.net may be more of an issue than finding php host, but that doesn't translate into to a 'better host' or more capable host, or even a more secure host.
I've worked with both types of sites, and each has their own set of positives and negative, but in the end, it comes down to return on investment (ROI). Just having a site up doesn't mean a thing until you start analyzing what impact (positive hopefully) it has on your business. The DNN framework alone doesn't give a site ROI, it's how it is used, its' flexibility, and administraion costs that proves the long term value. DNN is built to leverage the very capable server and .Net infrastructure, improves with each release, is supported globally, and has a growing list of ISP provider/hosts that recognize its' value.
A consultant has to say something, I know this because I do consulting, but it doesn't make consultants gods or fortunetellers. A good consultant will back up their report with facts, not generalities. Don't be dismayed or led to believe your work is anything less than the quality you give to it, and make sure you let your customer know that you stand by your choice of framework and technolgy. Do some homework, compile of list of performance statistics and business that use DNN and host it. Then, you can respond to what ever critique others throw at it.
That's my opinion, and I'm not really a MS die-hard. In fact, I'm back to using Unix and Linux because that's what the folks who pay me want, and it's really in support of a clininical information system not a web site. However, after working on DNN, and advocating its' adoption in house for 4 years, it appears that's we are going to finally move to using DNN for our web site. Of course, it helps that we are mainly an MS concentric site at this point.