Reduction of a ketone in the presence of an aldehyde
The best strategy is probably to chemoselectively protect the aldehyde before the borohydride reduction.  Protecting groups are derivatives of certain moieties in a molecule that are far less reactive under most conditions, but which can be easily removed under very specific conditions to regenerate the original moiety.

For carbonyl compounds -- this includes both aldehydes and ketones -- the most common protecting groups are all introduced by ketalization/acetalization.   Common examples are dimethyl acetals, 1,3-dioxolanes, and 1,3-dioxanes.

However, in general, aldehydes are far more prone to acetalization than ketones are to ketalization.  That offers a means to chemoselectively protect aldehydes.  Acetals such as 1,3-dioxanes are stable to borohydride reductions, but, as you note in your question, ketones are sensitive.

Poking around reveals several potential strategies for selective chemoprotection of an aldehyde.  Here are two examples.


Tetrabutylammonium Tribromide (TBATB) as An Efficient Generator of HBr for an Efficient Chemoselective Reagent for Acetalization of Carbonyl Compounds:


  This convenient, mild, chemoselective method allows acetalization of an aldehyde in the presence of ketone...

Chemoselective Protection of Aldehydes in the Presence of Ketones Using RuPVP Complex as a Heterogeneous Catalyst


Putting it all together, a recipe for selective borohydride reduction of a ketone in the presence of an aldehyde would be something like:


Protect the aldehyde.  The best choice is probably as a 1,3-dioxane acetal by reacting your compound with 4 equivalents of 1,3-propanediol, 1 equivalent of triethyl orthoformate, and 0.01 equivalent of tetrabutylammonium tribromide.  Under suitable conditions, the ratio of protected aldehyde (desired) to protected ketone (undesired) should be >50.
Borohydride reduction of the aldehyde-protected compound.  Only the unprotected ketone will react.
Deprotection.  Acetals can be removed in acidic conditions.  A number of catalysts that will effect deprotection without requiring acid have also been developed.