My sincerest apologies for any inconvenience. It seems the 550 does not, in fact, allow for favorite channels... perhaps that, in addition to it being end of life, is why it is being sold so cheap. I'm surprised no one corrected me regarding this.
You could still create sequences to select favorite channels; it will take a few more clicks but should still yield the desired effect. Click "Activities" tab. Beside "Watch TV", click "Customize Buttons". Click "Add Sequence" and follow through creating a sequence. Example: in the text box, write the sequence name as "Discovery Channel". From the drop down menus (first column), choose the device controlling your channels, such as TV or cable box. If you have to enter the channel as 052 then in the second column, select 0, 5, 2. You'd need three rows. If you normally enter the channel as 52 ENTER, then in the third column select 5, 2, ENTER (or SELECT, or NumberEnter).
You can then click "Add another sequence" to continue adding sequences of commands for favorite channels. I'm not sure that Harmony ever established an official maximum on sequences, but I wouldn't be surprised if one existed.
Once you have the sequences setup, click "Done" and map the buttons for your activity. You can map the favorite buttons to the LCD or any buttons on your remote. Now when you are mapping buttons, if you look in the drop downs in the second column there will be a choice called "Sequence" in addition to your devices. This is where you choose the sequences you've set up. You could use the sound/picture buttons for toggling favorites as well. There's a drop down box in the top left corner of the "Customize Buttons" screen where you can choose activity/sound/picture. If you press the sound or picture button on your remote, it toggles the respective mode of the remote. You could assign favorites sequences to numbers 0 thru 9 in sound mode as well as picture mode. For example, you start the TV activity, then press "Picture" to choose your favorites, then choose 0 to 9 for your channels (then press "Picture" again to return to your regular activity controls). Of course it would probably be simpler just to assign all the sequences to the LCD screen, as then you can see text labels for each channel that you've setup, but it's just another way of doing things for your consideration.
Make sure that you do not repeat the same sequence name between different activities (ie: don't make two sequences called "Discovery Channel" for two different activities... use unique sequence names, you can always rename the actual button mapping itself when adding the sequence to the activity's button mapping).
Compared to the Harmony, the URC MX-880 suggested by LessisNevermore is just slightly more expensive at around $500... probably overkill for your gf's TV.