If the employee was paid on a cumulative basis, then the "Copy for new employer" on the first P45 will have provided details of pay and tax to date to the new employer and these amounts would then be used by the new employer to work out the correct amount of tax on a year to date basis, and the pay from the first employment will be included in the figures at 7 on the second P45.
The figures at 8 on the second P45 will show details of wages paid and tax deducted in the second employment only.
If the employee was paid on a week/month 1 basis, the "Copy for new employer" on the first P45 would not have contained figures at 7 on the P45, but these details might have been received directly from HMRC.
If in the second employment the employee was paid on a week/month 1 basis, the second P45 will only have figures at 8 and in that case you need to add together the figures from both P45s. If on the other hand the employee was paid on a cumulative basis on the second employment, you need to take the figures from 7 on the second P45. I think the latter may be the case if your client says that the figures on the second P45 include pay from the first employment.
If you have problems understanding what went on you could look at the figures in conjunction with how much your client was paid throughout the year and information held on his/her online HMRC account.