I think that the most important usability dimension for this app is efficiency, in terms of entering the information, reading the information, and how fast it can do the calculations. However, learnability is also an important factor, as a convoluted app that is hard to use might push Trinity and Neo to continue using their questionable spreadsheet.
There are several inherent issues with using a spreadsheet to track these expenses. First, it is cluttered. There are 55 rows of expenses with everyone's expenses there and every expense is associated with only one category of spending. There doesn't seem to be an easy way to collapse all the irrelevant information. This means it takes longer to read since your eyes have to carefully slide over each row to make sure you have the right number. This severely affects the dimension of efficiency. A potential way to address this issue in the app would be to make the columns individual sections with all the corresponding expenses under them that are collapsable.
Another huge issue with their method of expense tracking is that there isn't a clear date section associated with each expense. Instead, they sometimes write the date next to the description. Sometimes, they might forget to log the expenses until later, so the expenses aren't chronological. Credit card payments are paid monthly and not having the option to easily see the date an expense was made severely affects efficiency and safety; they will have to find where the monthly expenses start and end and do those calculations and failure to make those payments will affect credit score! Adding a section to input the date in the app will help them both keep track of their expenses better.
Another issue is that the summary of the expenses are blended in with all of the actual expenses since there is no font size variation in the "sum" column. This impacts efficiency of using this spreadsheet since a lot of the times, users might be looking for that information in particular. As mentioned above, having different sections with the corresponding calculated expense total next to it would make this important more easily accessible to the user.
The last main problem I found while using this app is that sums are not automatically updated when a new entry is added. The user then has to go in and manually change the range of the rows summed to add the new expense. This severely decreases efficiency of using the app once again, as the user has to take extra steps to enter in information. This also affects safety since forgetting to make that change to range might result in an incorrect sum which can affect the credit card payment and whatnot. This issue should be addressed in the app where the sums should dynamically change as the data inputted changes.