All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is not permissible to invest money in government investment certificates and the like and this is Riba (interest and/or bribery) as we clarified in Fataawa 81911 and 84837.
Since this is the case, you should prevent your wife from investing money according to that system and clarify to her the prohibition of such an act and that what is earned from that investment is ill-gotten and evil. Also, you should clarify to her that there is no good or blessing in what is forbidden; Allaah Says (what means): {Allaah destroys interest.}[Quran 2:276]
Riba is also a reason for sorrow, unhappiness and having a miserable life, and it prevents the supplication from being answered. Indeed, every flesh that has grown from what is forbidden, the fire is more worthy of it as clarified in the Ahaadeeth.
Therefore, you should admonish your wife about this so that she would refrain from committing this forbidden matter and if she responds, all praise be to Allaah, otherwise, she bears the responsibility for her sinful act and you have done what is required from you. Allaah Says (what means): {…and no bearer of burdens will bear the burden of another.}[Quran 6:164]
Allaah Knows best.