Do you have any recommandation ... for this precise version ?
Yes... Upgrade !
Forms 6i (Developer 6i) was desupported a long time ago (Jan 2005). Microsoft hadn't even named Windows 7 at that time. Take a look at MyOracleSupport note 228666.1
I do not recommend trying to install Forms/Reports 6.x on Windows 7 for a variety of reasons, unless it is your own environment and you realize that you will likely have problems with it.
I (as the Forms Product Manager) would prefer to see you upgrade your customer to a current release like 126.96.36.199, but if you/they must use this very old version, you might be able to have some success using Windows XP Mode installed in your Windows 7 environment. This wasn't exactly considered certified or supported either, but it should work and since 6i isn't supported any longer, you really don't have many options. This will only work with Windows 7 Professional or higher. Refer to the Microsoft website for more information.