In that case (using Excel for an email list), installing an email client would probably be the best solution. By installing an email program, i.e. client, Windows has an email-specific program to work with, a program it can recognize as the program needed when an email address in Excel, Word, etc. is clicked on. Windows 7 did not come from Microsoft with a bundled email client, unlike previous version of Windows which had Outlook Express or Windows Mail (Windows Vista). But, a free email client called Windows Live Mail can be downloaded from Microsoft. A non-Microsoft alternative is an email client called Thunderbird, from Mozilla, the folks the make the Firefox browser.
Windows Live Mail is part of a larger package called
Windows Essentials. Thunderbird can be downloaded from
https://www.mozilla.org/en-US/thunderbird/download. Since your SIL is using MS Office Home and Student, I'm a bit inclined to recommend Windows Live Mail over Thunderbird.
If an email client is installed, the email account(s) will have to be set up in the email client. In other words, simply installing the email client will not allow it to immediately start handling email. It does not know what email accounts you have until you tell it, i.e. setup the email account. And, if she starts using an email client and continues to access the same email account via webmail (web browser), she needs to understand that sending email from webmail access will not cause those messages to appear in the email client since it is a separate program, assuming the email account is setup in the email client as a POP-type email account, which is quite common. So, using the email client for all email activity might be the best approach, at least while she's involved in planning he HS class reunion.
