This might work by adding a second table occurrence of your measurements table and relating it to the original one If you relate it using the date fields and show only the date higher then the current record.s date then you might be able to get the date from the record above in the table occurrence that you can then use to calculate the difference.
The relationship would be between the date fields and would be a "is greater than" type.
Then you can create calc fields in your measurements table that subtract the weight from the Table occurrence from the weight in the measurements table.
This is just off the top of my head, but I think this should work.
It seems to me that since a client will be measured multiple times, that each set of measurements needs to be its own record in a separate table related to the client table. Is that what you mean by your Measurements Table? The record could simply have the client's ID as a foreign key, a date on which the measurements were taken, the measurements themselves, and perhaps any other relevant notes (the client and been sick for a week before, the day was exceptionally hot, etc). Then comparing progress would be relatively simple because all measurements could be shown in a portal on the client's record.
What you are saying is correct keywords, but that's the basic initial setup. Which I think he already has.
A client table with a related measurements table.
The tricky part is in comparing a measurement to a previous measurement. And that's why I suggested to make a second table occurrence and to relate it using a date to define the relationship. Then a calculation can be made using the the tables occurrences to compare the measurements.