I own a non-MAC. I sent several .TIFs to an email address. Owner of email address also has MAC and tried to open them. I have them as 200Kb-300Kb each in size. She claims that they are 90Mb each and cannot open them. She is new user and gets frustrated very easily. What is she doing wrong?

Mark, this is sort of like saying, "My car won't start. Why?"

Way too many things could be causing this... and it may NOT be on her end.
BOTH file sizes seem off. 200k for a tiff seems small, and of course 90mb seems a bit much. The first obvious thing is that it's all but impossible that she is receiving A 90mb file, much less several. No ISP gives you that kind of mail box space.

Are either of you in some type of large network (inside an office network). Many large business' will be behind a firewall's and have gateways. I've seen many instances of corrupt files coming from behind these kinds of networks. If the files she is recieving has a "blank generic" icon, then you can bet the resource got scrambled. Some times you can correct this by forcing an app to open the file, but sometimes the image is damaged.

Another questions is what kind of email apps are you guys using? Are either of you using web based type email (hotmail or yahoo)? What kind of encoding are you using when sending the file?

My guess is that the file is being corrupted in transport. This could be caused on either end, so don't be too fast to blame her inexperience.

My first recommendation would be to resend the files, but one at a time. Keep it simple until you figure out where the problem is. If you can't find the problem, then try zip/stuffing the file before transporting and see what happens.
This in itself sounds simple, but it's not, unless you understand a few basics of cross-platforms.
Pc's use "zipping", while Mac's use "stuffing"....two different methods of compression. She should be able to "unzip" your file as long as she has "Stuffit" installed.
Remember that if she sends something to you, chances are it will be "stuffed" NOT 'zipped'. While both platforms can handle either types of compression with the right software installed, you both should agree on what method of compression you'll use to keep in uniform.

Generally speaking, image formats are "universal" in the respect that either computer platform will open these files, as long as you have an application installed to handle it (Photoshop, etc).
If she is not sure if she has an app, she can always using Internet Explorer to open any kind of image file. She can "force" the file to open via Explorer by simply dragging the file on to the Explorer icon.