Yes, she can use Messages on her Mac to text to anyone with an email address or phone number. Here's and Apple page on how to setup Messages on her Mac. She has to make sure her Mac has an active internet connection (you might need to check that for her first). You didn't mention the type of iMac she has so this assumes she has the latest operating system.
It's very simple, she just needs to start a conversation in Messages and then add your son's Android phone number or its email address (every internet-enabled phone will have an email address). She'll be able to chat with him after that.
This is called sending an SMS message.
- Click at the top of the Messages
- Address your message by typing the recipient's name, email address, or phone number. Or click plus sign and choose a recipient. If they're in your Contacts app, Messages finds them as you type. To start a group conversation, just add more recipients.
- Type your message in the field at the bottom of the window, then press Return to send it.